Spanish prime minister José Luis Rodríguez Zapatero made a direct appeal for intervention by the European Union and the European Central Bank (ECB) on Thursday as the country's borrowing costs soared to levels widely considered to be unsustainable.

Referring to the sovereign powers ceded to those European institutions since Spain joined the euro club, Zapatero said: "That is why power has been transferred to them." His request appeared to have been answered by late in the day as pressure on Spanish bond yields relaxed amid reports that the ECB was buying Spanish debt. In the meantime, pressure was piling up on Mariano Rajoy, the People's party (PP) leader expected to take over as prime minister after Sunday's general election, to reveal his plans for saving the country from a bailout that might bring eviction from the eurozone.

Rajoy remained tight-lipped, however, as Spain's treasury was forced to borrow money at a rate of almost 7% on Thursday for the first time since 1997, declining to give further details of what is expected to be a major reform and austerity programme.

"I do not have a magic wand to fix these problems, nor can we expect that they will all be solved in one day," El País quoted the conservative leader as saying at a campaign rally.

In an interview with El País published on Wednesday morning, Rajoy refused to say who his finance minister would be, arguing that he had not yet told the person in question, even though he himself had already decided.

He said there was no need to start asking the future minister to prepare emergency plans, as the party already has clear ideas about what it was going to do. "Both investors and a majority of Spaniards want change, political change, and that is the first thing that will generate confidence," he claimed.

Rumours continued to circulate, however, that the PP was drawing up a programme of dramatic reforms that would be implemented in the first month of a new government.

Those reforms would involve everything from the labour market to the country's financial sector, which is weighed down with debt accumulated as Spain inflated a residential housing construction bubble that burst three years ago.

With the outgoing Spanish government lowering its growth predictions for the year to 0.8% on Wednesday and admitting that the economy had stopped growing after the second quarter, Rajoy will have to make serious cuts or raise taxes if he is to meet the 4.4% deficit target set by the EU for next year.

He has campaigned on tax cuts, though many economists think he will have no option but to raise them overall to begin with, especially after the European commission said that it thought Spain would miss this year's deficit target of 6%.

It will require an estimated €17bn (£14.5bn) of cuts to meet that target if revenues not boosted by either growth or tax rises.

Rajoy said that he still aimed to lower taxes on small businesses so that they could create job in a country where unemployment has hit 23% and is still rising.

He confirmed that his government would fight to stop the eurozone splitting into two or more parts. "The idea of a two-speed Europe seems ridiculous to me," he told El País.

Latest polls show that the PP is some 15% ahead of Zapatero's socialists, which should give Rajoy an absolute majority in parliament and the freedom to introduce undiluted reforms quickly.

Arturo Fernandez, vice-president of Spain's employers federation, warned on Thursday that the country's bond yields were dangerously close to a level that he called "unsustainable".

With neighbouring Portugal and fellow southern European economy Greece both needing bailouts after their 10-year bond yields rose above 7%, Fernandez said that he hoped the new government would be able to clear doubts about the country's future. "What no one wants is a bailout," he said.